Fix build for ocp-build + external packages
Include Uutf-0.9.2 + Jsonm-0.9.1 in a single OpamJson module
Update OpamProcess interface to have a structured info field
Add a --output-json command line option to output the result of an OPAM command as a structured output.

Currently, the JSON file only contains error reports.
Merge branch 'master' of github.com:OCamlPro/opam